X‐Ray Observations of BL Lacertae during the 1997 Outburst and Association with Quasar‐like Characteristics

This paper reports on the X-ray emission from BL Lacertae during its 1997 July outburst as observed with the Rossi X-Ray Timing Explorer ( RXTE ), compares the RXTE data with previous measurements, and interprets the overall electromagnetic emission in the context of the currently popular theoretical models. The source is bright and variable, with the 2-10 keV flux approximately 2-3.5 times higher than measured in 1995 November by ASCA . The spectrum is also harder, with power-law energy indices of ~0.4-0.6, compared with ~0.9 in 1995 November. Both in the optical band, where BL Lacertae now shows broad emission lines, and in the X-ray band, where the spectrum is hard, the overall electromagnetic distribution of BL Lacertae is similar to that observed in blazars associated with quasars rather than to that seen in the more common high-energy peaked BL Lac-type objects. We argue that the high-energy (X-ray and γ-ray) emission from BL Lacertae consists of two spectral components: X-rays are produced by Comptonization of synchrotron radiation, while the γ-rays are produced by Comptonization of the broad emission line flux.